Rotor angle estimation for permanent magnet synchronous motor drive

ABSTRACT

A method of determining a rotor angle in a drive control for a motor, comprising the steps of (a) determining a rotor magnetic flux in the motor; (b) estimating the rotor angle on the basis of the rotor magnetic flux; and (c) correcting the estimated rotor angle on the basis of reactive power input to the motor. Step (a) may include the step of non-ideal integration of stator voltage and current values. 
     Step (b) may include the step of correcting phase errors caused by said non-ideal integration via a PLL circuit with phase compensation (F). Step (c) may include the steps of (1) calculating a first reactive power input value as 1.5*We*(C_Lq*I*I) and a second reactive power input value as 1.5*(Vq*id−Vd*iq); (2) determining a difference between said first and second reactive power input values; and (3) applying said difference to the rotor angle estimated in step (b) to obtain a corrected rotor angle.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of application Ser. No. 10/294,201filed Nov. 12, 2002 now U.S. Pat. No. 6,910,389 entitled ROTOR ANGLEESTIMATION FOR PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE, whichapplication is based on and claims priority of U.S. Provisional PatentApplication Ser. No. 60/337,506 filed Nov. 12, 2001, the disclosures ofwhich are incorporated by reference.

FIELD OF THE INVENTION

This invention relates to controls for motor drives and morespecifically relates to a technique for the estimation of rotor angle ina permanent magnet synchronous motor drive.

BACKGROUND OF THE INVENTION

Rotor position information is in general required for the stableoperation of permanent magnet AC motors having sinusoidal currentexcitation. Continuous rotor position has been obtained in the past fromencoders mounted on the motor shaft or indirectly through estimationalgorithms based on voltage and current feedback. The latter ispreferred because it results in lower system and operating cost.

However, most passive rotor estimation schemes (based on measuredvoltage and current) are complex and require precise knowledge of themotor parameters such as resistance and inductance. However, theseparameters, particularly the stator resistance, change widely withtemperature. This leads to inaccuracy in rotor angle estimation andresults in control stability problems, reduced torque per amperecapability and degradation of motor operating efficiency.

It would therefore be desirable to produce a rotor angle estimationscheme which provides maximum torque per ampere performance withoutrequiring accurate knowledge of the stator resistance or other motorparameters.

BRIEF SUMMARY OF THE INVENTION

The invention provides a novel method of estimating rotor angleinformation for the control of a permanent magnet AC motor havingsinusoidal back EMF.

The rotor angle is estimated via a phase-lock loop (with phase errorcompensation) which receives an estimate of the rotor magnetic flux. Therotor magnetic flux is obtained from the stator voltage (actual voltageor command voltage), current, resistance and inductance.

Then, the rotor angle estimation error (stator resistance change due totemperature) is removed by using a novel angle error corrector. Thiscorrector is based on reactive power compensation and is insensitive toresistance change. Furthermore, only one inductance parameter isrequired for the angle corrector's reference model.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a PMSM control system which includesan embodiment of the invention.

FIG. 2 is a more detailed block diagram showing the rotor angleestimator of FIG. 1.

FIG. 3 is a circuit diagram of a rotor magnetic flux estimatorassociated with the diagram of FIG. 2.

FIG. 4 is a more detailed diagram showing the rotor angle corrector ofFIG. 1.

FIG. 5 is a graph showing a relationship between reactive power errorvs. rotor angle error, per unitized to the motor rated power.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention as described in FIGS. 1 to 5 is related to a motorcontrol algorithm that is implemented in firmware. However, the scope ofthe invention includes implementations in any combination of hardware,firmware and software that would have been within the ordinary level ofskill in the art.

A block diagram of the control method is shown in FIG. 1. The d-axis isthe orientation which aligns with the magnetic axis of the rotor (theconvention used in the literature).

The following are the definitions of the quantities listed in FIG. 1.

id* flux current command iq* torque current command id flux currentfeedback iq torque current feedback ia, ib phase currents Rtr_Angestimated rotor angle C_Rs stator per phase resistance Del_Angcompensation angle from angle corrector Vab, Vbc line voltage feedbacksVd flux-axis voltage feedback Vq torque-axis voltage feedback Weinverter fundamental frequency

The rotor angle estimation block of FIG. 1 is shown in detail in FIG. 2.The inputs Flx_A and Flx_B are rotor magnetic fluxes which are obtainedby non-ideal integration of motor back emf which is formed by the statorcurrent, voltage, resistance and inductance as shown in FIG. 3. In theFigures, Tf represents the time constant of the non-ideal integrator.

It will be noted that the inputs (V_A, V_B, I_A and I_B) to the fluxestimator of FIG. 3 are simply the 3-phase (ia, ib, Vab, Vbc) to 2-phasetransformed signals.

The rotor angle estimator (FIG. 2) utilizes a novel flux phase lock loopsystem. A frequency feedforward circuit F compensates for phase errorsdue to the non-ideal integration of stator voltages which was used inFIG. 3 to obtain the flux. The phase error generated by the non-idealintegration is fully compensated for in the circuit F.

Then, the estimation error due to resistance is compensated by a rotorangle corrector system which is described below in connection with FIG.4.

The rotor angle corrector circuit of FIG. 1 is shown in detail in FIG.4. When the estimated rotor angle (FIG. 1) matches up with the actualrotor angle, a reference value for the reactive power (Q) input to themotor is equal to:1.5*We*(C_Lq*I*I+Flx_M*id+(C_Ld−C_Lq)*id*id)

Note, however, that for a permanent magnet surface mount (PMSM) motorthe airgap reluctance is identical in the d-axis and the q-axis. Thus,id=0 and Ld=Lq. Therefore, the above equation for reference reactivepower can be reduced to:1.5*We*(C_Lq*I*I)

The actual motor reactive power (Q), expressed in terms of voltage andcurrent only, is then computed by:Q=1.5*(Vq*id−Vd*iq).In the foregoing equations:

-   -   C_Ld—d-axis inductance,    -   C_Lq—q-axis inductance,    -   I—Stator current magnitude,    -   Flx_M—Equivalent flux linkage of rotor magnet,    -   Q—Terminal reactive power, and    -   We (omega e)—stator fundamental frequency.

Since C_Ld=C_Lq, the rotor angle correction can be achieved with onlyone inductance parameter (Lq or Ld). Lq is used in this case. Of course,the invention is adapted for use with other motor types as well, such asinterior permanent magnet motors in which Ld is not equal to Lq, as willbe appreciated by those having the ordinary level of skill in the art.

If the estimated rotor angle matches up with the actual rotor angle thenthe following relationship will be satisfied:(Vq*id−Vd*iq)−We*C _(—) Lq*I*I=0

Thus, the reactive power error between Q and (We*C_Lq*I*I) (the verticalaxis in FIG. 5) can be used to null out any rotor angle error (thehorizontal axis in FIG. 5), such that the maximum torque per ampere canbe maintained, even when there is an error in the resistance parameterused in the magnetic flux estimator (FIG. 3).

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention is not limited by the specificdisclosure herein.

1. A method of determining a rotor angle in a drive control for a motor,comprising the steps of: estimating a rotor angle; and correcting theestimated rotor angle on the basis of reactive power input to the motor.2. The method of claim 1, further comprising the steps of: a)determining a rotor magnetic flux in the motor; and b) estimating therotor angle on the basis of the rotor magnetic flux.
 3. The method ofclaim 2, wherein step (a) includes the step of non-ideal integration ofstator voltage and current values.
 4. The method of claim 3, whereinstep (b) includes the step of correcting phase errors caused by saidnon-ideal integration via a PLL circuit with phase compensation (F). 5.The method of claim 3, wherein step (a) includes the step of non-idealintegration of motor back emf based on stator resistance, inductance,voltage and current values.
 6. The method of claim 2, wherein said motoris an interior permanent magnet motor.
 7. The method of claim 2, whereinsaid rotor magnetic flux is determined without a filter function.
 8. Themethod of claim 1, wherein said motor is an interior permanent magnetmotor.
 9. The system of claim 1, wherein said reactive power input tothe motor is determined on the basis of motor frequency.
 10. A systemfor determining a rotor angle in a drive control for a motor,comprising: a circuit for estimating a rotor angle; and a circuit forcorrecting the estimated rotor angle on the basis of reactive powerinput to the motor.
 11. The system of claim 10, further comprising: a) afirst circuit for determining a rotor magnetic flux in the motor; and b)a second circuit for estimating the rotor angle on the basis of therotor magnetic flux.
 12. The system of claim 11, wherein said firstcircuit carries out non-ideal integration of stator voltage and currentvalues.
 13. The system of claim 12, wherein said second circuit correctsphase errors caused by said non-ideal integration via a PLL circuit withphase compensation (F).
 14. The system of claim 12, wherein said firstcircuit carries out non-ideal integration of motor back emf based onstator resistance, inductance, voltage and current values.
 15. Thesystem of claim 11, wherein said system is connected to a terminal ofthe motor for measuring said reactive power input.
 16. The system ofclaim 15, wherein said motor is a permanent magnet surface-mount motor.17. The system of claim 11, wherein said motor is an interior permanentmagnet motor.
 18. The system of claim 11, wherein said rotor magneticflux is determined without a filter function.
 19. The system of claim10, wherein said system is connected to a terminal of said motor formeasuring said rotor magnetic flux.
 20. The system of claim 19, whereinsaid motor is a permanent magnet surface-mount motor.
 21. The system ofclaim 10, wherein said system is connected to a terminal of the motorfor measuring said stator voltage current values.
 22. The system ofclaim 21, wherein said motor is a permanent magnet surface-mount motor.23. The system of claim 10, wherein said system is connected to aterminal of the motor for measuring said reactive power input.
 24. Thesystem of claim 10, wherein said motor is an interior permanent magnetmotor.
 25. The system of claim 10, wherein said reactive power input tothe motor is determined on the basis of motor frequency.